Magnetically soft materials, i.e., materials which typically exhibit macroscopic ferromagnetism only when a magnetic field is applied, find application in a great variety of technological fields. Exemplary uses are in heavy-current engineering, transductor cores, relays, inductance coils, transformers, and variable reluctance devices. Although many materials are soft magnets, this invention is concerned only with magnetically soft iron-nickel (Fe-Ni) alloys, and in particular, Fe-rich essentially ferritic alloys, and the discussion will be restricted accordingly.
The Fe-Ni alloy system offers a large number of technically important magnetically soft compositions, typically having compositions in the range 30-80 weight percent Ni. See for instance C. W. Chen, Magnetism and Metallurgy of Soft Magnetic Materials, North-Holland Publishing Co., 1977, page 389. Alloys in this compositional range have the austenitic (face-centered cubic, fcc) crystal structure. M. Hansen, Constitution of Binary Alloys, 2nd ed., McGraw-Hill, (1958), pp. 677-684, incorporated herein by reference.
In Fe-Ni alloys within the compositional range from 0 to about 20 weight percent Ni, the body centered cubic (bcc) lattice configuration prevails, and within the range of from about 20 to about 30 percent Ni, after normal cooling from the .gamma.-region to room temperature, a two-phase structure containing both a bcc and an fcc phase typically exists.
As a general rule, for soft magnetic materials the final product should be a single-phase solid solution in the equilibrium state, (W. Chen, op. cit. page 267). In agreement with this rule, the above two-phase region, i.e., the region from about 20 to 30 percent Ni, is usually not of magnetic interest. However, alloys near 30 percent Ni in the single-phase fcc region find application as temperature compensators.
In the prior art, Fe-Ni alloys having the compositional range 0 to 20 weight percent Ni have not found significant use, although their properties have been measured and published. See, for instance, R. M. Bozorth, Ferromagnetism, Van Nostrand, 1951, especially pp. 102-119, and G. Y. Chin and J. H. Wernick, Ferromagnetic Materials, Vol. 2, E. P. Wohlfarth, editor, North-Holland Publishing Co., (1980), especially pp. 123-168. The neglect of alloys in this compositional range can be explained by their technologically relatively unattractive magnetic characteristics, such as, for instance, their relatively low maximum permeability and relatively high coercive force, as exemplified by the prior art data referred to above. However, alloys in this compositional range have low material costs, and furthermore, supplies for Fe and Ni are substantially assured. Thus, Fe-Ni alloys containing less than about 20 weight percent Ni could be of considerable commercial value if their magnetic properties could be sufficiently improved.
An established soft magnetic material, used for instance as a ring armature in telephone receivers, is 2V-Permendur (49 percent Fe, 49 percent Co, 2 percent V). But the high cost and uncertain supply status of Co make development of a Co-free substitute material for this and other high-Co alloys desirable.